The present invention relates to the field of control of vehicle transmissions which are switchable between two wheel drive operation and four wheel drive operation, and in particular to the field of such control systems which operate automatically to switch the transmissions between a plurality of speed stages according to vehicle operational conditions.
Automatic transmissions for vehicles are per se known: such an automatic transmission provides one or the other of various speed stages (i.e. gearing ratios) between the vehicle engine and the wheels thereof, according to various operational conditions of the vehicle such as engine load and vehicle road speed, as well as according to the set position of a manual range setting means such as a lever which is set by the driver to one of several operational ranges such as "D" range, "2" range, and "L" range. The switching pattern between these speed stages is typically described by a shift pattern diagram.
Now, recently vehicles which can be optionally switched between a two wheel drive mode of operation and a four wheel drive mode have become more and more popular. Such a vehicle typically incorporates in its power train, typically after the main transmission unit thereof, a transfer transmission unit which can be switched either to provide power to the rear wheels only of the vehicle or to provide power to all the four wheels of the vehicle.
It might be conceived of to combine these two forms of transmission, in other words, to make the main transmission of such a two/four wheel drive type of vehicle be an automatic transmission unit. However, because the vehicle running characteristics are very much different between the four wheel drive operational mode of the vehicle and the two wheel drive operation mode, this has not heretofore been practicable, because a shifting characteristic for such a main transmission unit which has been suitable for use during two wheel drive operation has been quite unsuitable during four wheel drive operation, and vice versa. Accordingly up till now it has been effectively impracticable to apply an automatic transmission to a dual two/four wheel drive type of vehicle.